This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of any prior art.
As the world's petroleum supplies continue to diminish there is a growing need for alternative materials that can be substituted for various petroleum products, particularly transportation fuels. In the U.S., environmental regulations, such as the Clean Air Act of 1990, provide incentives for the use of oxygenated fuels in automobiles. Ethanol or methyl tertiary butyl ether (MTBE) boosts the oxygen content in gasoline and reduces carbon monoxide emissions. One principal advantage for the use of ethanol is that the fuel is produced from renewable resources. Atmospheric levels of carbon dioxide, a greenhouse gas, can be decreased by replacing fossil fuels with renewable fuels.
Currently much effort is underway to produce bioethanol that is derived from renewable biomass materials, such as corn, sugar crops, energy crops, and solid waste. Conventional ethanol production from corn typically competes with valuable food resources, which can be further amplified by increasingly more severe climate conditions, such as droughts and floods, which negatively impact the amount of crop harvested every year. The competition from conventional ethanol production can drive up food prices. While other crops have served as the biomass material for ethanol production, they usually are not suitable for global implementations due to the climate requirements of such crops. For instance, ethanol can also be efficiently produced from sugar cane, but only in certain areas of the world, such as Brazil, that have a climate that can support near-year-round harvest.
While other approaches of producing ethanol that do not use corn are available, they are still lacking. For example, Henk and Linden at Colorado State University investigated solid-state production of ethanol from sorghum (see Solid State Production of Ethanol from Sorghum, Linda L. Henk and James C. Linden, Applied Biochemistry and Biotechnology, Vol. 57/58, 1996, pp. 489-501). They noted for sweet sorghum to be used successfully for ethanol production, three issues needed to be addressed:                Carbohydrate storage;        Accessibility of the ligno-cellulosic fraction to enzymatic hydrolysis of hemicellulose and cellulose; and        A more economical means of recovering the ethanol from the sweet sorghum.        
In their process, they pointed out that seasonal availability and storability of sweet sorghum are important factors in the use of this renewable biomass. Sugar extraction and storability are two serious problems that have limited the use of sweet sorghum as a substrate for ethanol production. Traditional applications envision using juice containing about 10-15% sugar that has been extracted or pressed from the sweet sorghum pulp. The juice is then either fermented directly to alcohol or evaporated to molasses for storage. Direct fermentation of the juice to ethanol is a seasonal process, accomplished for only a short time after harvest. This presents challenges to scaling up solid state fermentation from an experimental stage to a larger practical stage, such as industrial scale. For example, the short harvesting window requires a substantial capital investment of storage space and recovery facilities to process the peak amount for a short period time while the space and equipment would sit dormant or be under-utilized for the down time.
Henk and Linden's strategy to some of the problems of making sweet sorghum to ethanol was to investigate using wet stored solid state fermentation integrated into an economical method for long-term storage of ethanol in sweet sorghum. While Henk and Linden did show some improvements in the overall process, there are still a number of shortcomings, including the amount of ethanol they were able to produce. Also, they did not provide an economic way to recover the ethanol from the solid state fermentation. Such proposed systems tend to make bioethanol production even more expensive by typically requiring expensive equipment that needs costly maintenance.
Others have also recognized challenges to economically recover the ethanol and other volatile organic compounds from the biomass solid material. For instance, Webster et al. reported that using a forage harvester for sweet sorghum results in rapid juice deterioration and therefore not an attractive solution for bringing in sweet sorghum to sugar mills (see Observations of the Harvesting, Transporting and Trial Crushing of Sweet Sorghum in a Sugar Mill, Webster, A., et al, 2004 Conference of the Australian Society of Sugar Cane Technologist, Brisbane, Queensland, Australia (May 2004)). Andrzejewski and Eggleston reported that challenges in making U.S. sweet sorghum to ethanol (or other uses) viable revolve around the storage of the juice because of the relatively narrow harvest window of sweet sorghum in the United States (see Development of commercially viable processing technologies for sweet sorghum at USDA-ARS-Southern Regional Research Center in New Orleans, Andrzejewski and Eggleston, Sweet Sorghum Ethanol Conference, Jan. 26, 2012). In particular, the challenges include (i) clarification (removal of suspended and turbid particles) of the raw juice to make it suitable for concentration and/or fermentation, (ii) stabilization of juice or partially concentrated juice for cost-effective seasonal use (liquid feedstock), and (iii) concentration of juice into syrup for storage, year-round supply, and efficient transport (liquid feedstock).
Bellmer sought to improve the process by optimizing conditions around removing the juice from the solids before processing (see The untapped potential of Sweet Sorghum as a Bioenergy Feedstock, Bellmer, D., Sweet Sorghum Ethanol Conference, Jan. 26, 2012). Wu et al. recognized the technical challenges of using sweet sorghum for biofuels, including a short harvest period for highest sugar content, and fast sugar degradation during storage (see Features of sweet sorghum juice and their performance in ethanol fermentation, X. Wu et al., Industrial Crops and Products 31: 164-170, 2010). In particular, the study showed that as much as 20% of the fermentable sugars can be lost in 3 days. Bennet and Annex noted the limitations of using sorghum for ethanol production involving material transport cost and storability (see Farm-gate productions costs of sweet sorghum as a bioethanol feedstock, Transactions of the American Society of Agricultural and Biological Engineers, Vol. 51(2):603-613, 2008). While Bennet and Annex were aware of direct production of ethanol in ensilage inoculated with yeast, they concluded that such direct production method was impractical because of issues related to separating ethanol from silage, ensilage storage losses (up to 40% in bunker style silos), and the possible use of silage as an alternative fermentation feedstock have yet to be examined for industrial-scale applications.
Shen and Liu sought to address the long-time and effective storage of fresh stalk or juice by first dried the sweet sorghum in order to preserve the sugars, then plan to use the material year-round for ethanol production, thereby adding costs of material handling for drying, spreading the wet sorghum for drying, as well as adding restrictions to the process by requiring adequate weather conditions to achieve proper drying (see Research on Solid-State Ethanol Fermentation Using Dry Sweet Sorghum Stalk Particles with Active Dry Yeast, Shen, Fei and Liu, R., Energy & Fuels, 2009, Vol. 23, pgs. 519-525). Imam and Capareda sought to process the juice before fermentation and to increase the fermentation rates using a variety of options such as autoclave (heat treat), freeze, and to increase the sugar concentration (see Ethanol Fermentation from Sweet Sorghum Juice, Imam, T. and Capareda, S., ASABE, 2010 ASABE Annual International Meeting, Pittsburgh, Pa. (June 2010)).
Bellmer, Huhnke, and Godsey noted challenges to using sorghum in ethanol production as: (i) storability of carbohydrates in sweet sorghum, (ii) quick sugar/carbohydrate degradation in-stalk after harvest, (iii) short shelf life of expressed juice, (iv) syrup production (dewatering) too costly (see The untapped potential of sweet sorghum as a bioenergy feedstock, Bellmer, D., Huhnke, R., and Godsey, C., Biofuels 1(4) 563-573, 2010). They used a solid phase fermenter, which are metallic containers including rotary drums and screw augers, which require expensive equipment. Further, use of a solid phase fermenter is also subject to the harvest window of the crop, e.g., sweet sorghum. Likewise, Noah and Linden noted storability and inefficient sugar extraction as the two major drawbacks to sweet sorghum use for fuels and chemicals.
In summary, obstacles in using sorghum and other plants containing fermentable sugars include the fact that they are only seasonally available and storage is costly, which make it challenging to use infrastructure efficiently and to schedule labor; sugar extraction and storability are two critical obstacles because conversion must be started immediately after harvest to avoid spoilage.
Thus, there is still a need for a process to economically produce ethanol and other volatile organic compounds from biomass material that addresses at least these obstacles, such as preferably not competing with the world's food source.